1. Field of the Invention
This invention relates to a method and apparatus for regulating energy consumption by a steam-fired absorption chiller. More particularly, this invention relates to a method for adjusting the steam control valve of a steam-fired absorption chiller so as to minimize or eliminate fluctuations in steam flow rate which are common in conventional steam-fired absorption chillers.
2. Description of Prior Art
Absorption chiller systems are well established in the prior art. In such systems, an absorbent-refrigerant solution from an absorber is heated in a generator to produce an absorbent-refrigerant vapor. The absorbent-refrigerant vapor is separated and condensed, producing a substantially pure liquid refrigerant and an absorbent-enriched solution, that is, a regenerated absorbent-refrigerant solution. The absorbent-enriched solution is circulated through a heat exchanger in heat exchange relationship with the absorbent-refrigerant solution from the absorber, preheating the absorbent-refrigerant solution before its introduction into the generator and producing a reduced temperature absorbent-enriched solution, which reduced temperature absorbent-enriched solution is returned to the absorber. The substantially pure liquid refrigerant is evaporated, producing a substantially pure refrigerant vapor, and the substantially pure refrigerant vapor is absorbed into the reduced temperature absorbent-enriched solution in the absorber, producing the absorbent-refrigerant solution.
In absorption chillers, there are three common energy sources for heating the generator-direct fired, hot water, and steam-fired. Energy supplied to the generator concentrates the absorbent solution and separates out refrigerant. The amount of energy required is dependent upon the absorbent-refrigerant solution concentration entering the generator, the solution flow, and the desired concentration of the absorbent-enriched solution leaving the generator. For steam-fired chillers, energy is obtained from the phase transformation of steam from vapor to liquid. The heat of vaporization is transferred to the cooler absorbent-refrigerant solution in the generator by conduction through generator tube surfaces. Energy transferred is proportional to steam flow rate. Steam flow into the generator is a function of supply pressure, saturation pressure in the generator, and steam valve position.
The challenge in controlling a steam-fired absorption chiller is in the regulation of steam flow to the generator. Steam supply pressure is usually held constant by a steam regulator; however, there are cases when supply pressure fluctuates significantly. On the downstream side of the steam control valve, steam saturation pressure in the generator is always changing. This pressure varies the chiller capacity and is very low at startup and when the chiller is running with cold cooling water temperatures. The result of low generator pressure is that the differential pressure across the steam control valve is large. When this pressure differential is large, it is possible to get steam flows greater than design capacity. This, in turn, can cause two problems. The first problem is that the boiler supplying steam to the unit may not be capable of producing more than the designed steam flow rate. The second problem is that the excess steam flow may overfire the chiller.
Historically, empirical methods have been used to determine steam valve travel limits at startup and at low cooling water temperatures to limit steam flow. However, this method is not exact, and there is no compensation for fluctuating steam supply pressure.
U.S. Pat. No. 5,477,696 teaches a device for adjusting the opening degree of a burning quantity control valve which controls heat supplied to a generator in order to control the amount of heat provided to a solution therein during cooling cycle operation. The device includes detector means for sensing a temperature of the solution heated and concentrated in the generator, detector means for sensing the inlet or outlet temperature of chilled water flowing into or out of the evaporator, detector means for sensing the inlet or outlet temperature of cooling water which is fed from the absorber to the condenser, and detector means for picking up an opening degree of the burning quantity control valve which supplies the heat source to the generator to obtain a present heating quantity of the machine in operation.
U.S. Pat. No. 5,619,859 teaches a double effect absorption refrigeration unit which uses water as a refrigerant and an aqueous solution of lithium bromide as an absorbent solution, respectively, and comprises an evaporator, an absorber, a high temperature generator, a low temperature generator, and a condenser. The maximum permissible temperature of the high temperature generator is determined by an inlet temperature of the cooling water while the amount of heat is controlled according to variations of cooling load, such as an outlet temperature of cooled water. When the temperature in the high temperature generator exceeds the maximum permissible temperature as determined by the inlet temperature of the cooling water, the amount of heat primarily determined by the cooling load is reduced. The absorption refrigeration unit insures adequate control at the time of startup, achieves energy-saving by suppressing a surplus refrigerating capacity when the cooling water is low in temperature, and avoids the pressure rise in the high temperature generator, irrespective of the temperature of the cooling water, when an abnormality occurs, such as in the case of a large amount of hydrogen gas being generated.
U.S. Pat. No. 3,813,037 teaches a closed condensate system for steam in which the steam from a boiler is used in various applications including air conditioning, where all of the steam from the boiler is used and all of the condensate from the steam can be returned to the boiler. A closed cycle condensate receiver is used in the system and means are employed for maintaining a pressure within the receiver so that the temperature of the condensate returned to the receiver will always be lower than the corresponding pressure within the receiver, thereby assuring that the closed condensate system returns the condensate from the heat exchanging elements of the system back to the boiler without flash loss.
U.S. Pat. No. 4,187,543 teaches an apparatus for controlling the discharge temperature of thermal fluids presented from a common thermal source to one or more HVAC systems in a network whereby each HVAC controls the volume of thermal fluid circulating therethrough by regulating the valve position of the HVAC valve apparatus over a valve position range from a fully closed position to a fully open position dependent upon the thermal energy level required by one or more temperature regulated living spaces associated with each HVAC. The apparatus includes electronic processing means in combination with dedicated temperature and valve position sensing apparatus for providing actual position signals for each fluid valve apparatus and an actual source discharge temperature signal. The electronic processing means determines the magnitude of the greatest thermal energy demand within a short term interval dependent upon the most open of the actual valve position signals and provides modulation of the source discharge temperature over the short term interval dependent upon the magnitude of the greatest thermal energy demand.
A control system for transfer of energy to and from a working fluid in which a pressure sensor determines the condition of the working fluid which is then used to regulate a fuel burner is taught by U.S. Pat. No. 4,373,663. See also U.S. Pat. No. 4,577,280 which teaches a system for controlled distribution of steam from various pressure lines for energy management and/or for steam-power cogeneration in an industrial plant; U.S. Pat. No. 5,138,846 which teaches a system for controlling the cold water outlet temperature of an absorption refrigerator by fuzzy logic control; U.S. Pat. No. 5,224,352 which teaches a control device for an absorption refrigeration machine which maintains constant a liquid surface of a generator with respect to start and stop or abrupt variation of load of the absorption refrigeration machine using fuzzy logic control; U.S. Pat. No. 5,289,868 which teaches an absorption chiller heater and unit-type air conditioning system provided with load responsive type control means for performing cooling and heating operation control in accordance with a load; U.S. Pat. No. 5,423,189 which teaches a control system for an absorption heat transfer plant comprising means for controlling the supply of heat to the concentrator responsive to the temperature of the evaporator liquid refrigerant and means for controlling the temperature of a cooling fluid which cools the heat transfer plant's absorber and condenser and which maintains the cooling fluid at an optimum temperature as the cooling fluid enters the absorber; and U.S. Pat. No. 5,557,939 which teaches a control system for absorption chillers comprising a device for measuring operating state data concerning components of the main assembly of the chiller including the liquid level of a high temperature generator and the degree of opening of a gas valve, and a fuzzy control circuit for calculating the deviation of the liquid level from a target value based upon the measured operating state data, predicting a variation in the liquid level from the deviation and another item of the operating state data and calculating a control input as to the inverter frequency of an absorbent pump based upon the result of prediction, whereby, even in the event of a great disturbance, the liquid level of the generator can be controlled with high responsiveness to minimize the variation of the liquid level.